Etymologies

French : in-, not (from Old French; see in-1) + souciant, present participle of soucier, to trouble (from Old French, from Vulgar Latin *sollicītāre, alteration of Latin sollicitāre, to vex; see solicit).

(American Heritage® Dictionary of the English Language, Fourth Edition)

From French insouciant from in- (“not”) + souciant (“worrying”) (Wiktionary)

Examples

There's a wonderful cover shot to the 1958 album Legrand Jazz, with pianist-composer Michel Legrand wearing an expression of insouciant expectation, Gauloise at the corner of his mouth, indolently summoning invisible sidemen to action.

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Comments

Michael Azerrad, witing in 'Our Band Could Be Your Life' described the voice of Sonic Youth's Kim Gordon as "a sort of insouciantholler, like a kid calling to her friends about something great she'd found but trying not to seem too excited about it".